document.getElementById is a function that takes a string, finds the element in your document with that id, and returns that element. Here, you’ve selected two <input> elements, which the above snippet assigns to firstInput and secondInput to distinguish them (<input>s) from numbers.

Each input has a value property, which is a string. Verify this in your browser’s console.

console.log(firstInput.value); // whatever you typed in the first box
console.log(typeof firstInput.value); // string

Onwards to parsing, then. parseInt is a function that parses a string into a number. You can try it out in your console, too: